The present invention relates to a fault current detection system. More particularly, the present invention relates to a fault current detection system that is implemented via software and relates to a method employed by the detection system.
In many applications, electrical currents are supplied to one or more electric devices to provide power for the devices. For example, electrical currents are supplied from a power company to one or more electrical outlets in a residential home, and a user can connect an electric device to an outlet to supply power to the device. If the electric device malfunctions or is mishandled by the user, a potentially dangerous situation may arise. For example, if the user contacts a portion of the electric device that receives electrical currents from the power company, the electrical current will pass through the user to the ground and may cause the user""s heart to suffer from a cardiac arrest. Also, if the portion of the electric device that receives electrical currents is improperly grounded due to faulty insulation, a current will be supplied to the electric device and may start a fire in the user""s home. The additional surge of current that is supplied to the user""s home when the electric device malfunctions or is mishandled is known as a fault current.
In order to prevent fire in the user""s home or to prevent the user from being harmed, a circuit breaker has been developed that detects fault currents and that blocks the supply of electrical current to the one or more electrical outlets in the user""s home if the detected fault currents exceed certain levels. FIG. 7 shows an example of such a circuit breaker 1 that comprises a sum-current transformer 2, a power supply 4, a triggering circuit 5, a triggering relay 6, a switching mechanism 7, and a switch 8.
The electric currents are supplied from the power company to the user""s home via a conductor network LN, and the network LN includes three active conductors L1, L2, and L3 and a neutral or ground conductor N. The conductor network LN is wrapped around a core 3 of the sum-current transformer 2 to form a primary winding N1 of the transformer 2. Also, a secondary winding. N2 is wrapped around the core 3 of the transformer 2, and the triggering circuit 5 is connected to the winding N2. Specifically, the triggering circuit 5 is connected across the output terminals of the winding N2, and the triggering relay 6 is connected across the output terminals of the circuit 5. The triggering relay 6 controls the switching mechanism 7 to selectively open and close the switch 8, and the switch 8 is provided in the path of the conductor network LN between the power company and the electrical device.
When the electrical device in the user""s home is operating or being handled under normal conditions, no fault currents exist. As a result, the vector sum of the currents flowing through the core 3 via the conductor network LN is zero. However, if a fault current If is generated, the vector sum of the currents is not zero, and a voltage Ue is generated across the secondary winding N2. The characteristics of the voltage Ue correspond to the characteristics of the fault current If, and the triggering circuit 5 generates an output voltage Ua based on the input voltage Ue. The output voltage Ua causes a current Ia to flow through the triggering relay 6, and the relay 6 triggers. The triggering of the relay 6 causes the switching mechanism 7 to open the switch 8 and block the supply of current from the power company to at least one outlet in the user""s home. Accordingly, when the user contacts a conductive portion of an electric device connected to an outlet and causes a fault current If to be generated, the relay 6 triggers, and the switching mechanism 7 opens the switch 8. As a result, the dangerous fault current If is no longer supplied to the user""s home and will not harm the user.
The value of a triggering fault current Ixcex94trigger of the circuit breaker 1 (i.e. the value of a fault current If that will trigger the relay 6) is determined based on the rated residual current (or nominal fault current) Ixcex94n. The nominal fault current Ixcex94n corresponds to the sensitivity of the circuit breaker 1 and is selected based on the electrical standards of the electrical system in which the circuit breaker 1 is incorporated. An example of how the triggering current Ixcex94trigger is selected will be described below in conjunction with the graph illustrated in FIG. 8.
The graph shows an example of a fibrillation limit curve G1 and a fire prevention limit curve G2. The fibrillation limit curve G1 represents the maximum value of the fault current If that will not cause a user""s heart to fibrillate if the user contacts the current If, and the values in the curve G1 are,dependent upon the frequency of the fault current If. For example, if the fault current If has a frequency of 100 Hz and is less than or equal to about 30 mA, the user will not suffer ventricular fibrillation, but if the fault current If is greater than approximately 30 mA, the user will experience fibrillation. On the other hand, if the fault current If has a frequency of 1 kHz, the user""s heart will not fibrillate if the current If is less than or equal to approximately 420 mA but will fibrillate if the current If is greater than such value.
While the maximum current values in the fibrillation limit curve G1 are dependent on the frequency of the fault current If, the maximum current values represented by the fire prevention limit curve G2 are not frequency dependent. In particular, if the value of the fault current If (at any frequency) is less than or equal to approximately 420 mA, a fire will not occur in the user""s electric device or home, but if the value is greater than 420 mA, a fire will likely occur. In the present example, the current value of 420 mA is selected for a power system with a voltage of 230 V (with respect to ground) in order to prevent a power dissipation that is greater than 100 Watts at the location of the fault.
As indicated above, the specific values and characteristics of the limit curves G1 and G2 are governed by the electrical standards of a particular electrical system. For example, the limit curve G1 is determined according to the international standard IEC 479. If the circuit breaker 1 were operating according to different standards, the specific values of the curves G1 and G2 would be different.
The triggering fault current Ixcex94trigger, which causes the circuit breaker 1 to trip, should be selected based on both the fibrillation limit curve G1 and fire prevention limit curve G2 on the graph shown in FIG. 8. Specifically, the triggering fault current Ixcex94trigger should be selected such that, when a fault current If occurs, the circuit breaker 1 will trigger before the fault current If rises to a level that can cause injury to the user of an electric device or to a level that can cause a fire. Therefore, if the circuit breaker 1 is operating in an environment in which fault currents having low frequencies may be generated, the triggering fault current Ixcex94trigger may be set to a value that is below the fibrillation limit curve G1 at low frequencies. In the example shown in FIG. 8, the triggering fault current Ixcex94trigger would be less than approximately 30 mA if harmful fault currents If having frequencies of 50 Hz may possibly be generated. However, as shown in FIG. 8, the maximum values of the limit curve G1 significantly increase as the frequency of the fault currents If increases.
In addition, several types of fault currents If may occur that can cause harm to a user of an electric device or than can cause a fire in the user""s home. The different types of fault currents include an alternating fault current, a pulsating direct fault current, and a smooth direct fault current.
An alternating fault current occurs when the fault current If is an alternating signal and the magnitude of negative amplitude of the input signal is distorted with respect to the positive amplitude of the input signal. A smooth direct fault current occurs when the magnitude of the negative amplitude or the magnitude of the positive amplitude of the fault current If falls within a certain range of values around the effective value of the fault current If. A pulsating direct fault current occurs when the magnitude of the negative amplitude or the magnitude of the positive amplitude falls outside the certain range of values around the effective value of the fault current If.
Accordingly, the circuit breaker 1 should ideally detect whether or not a fault current If has occurred, what type of fault current If has occurred, and whether or not the particular type of fault current If is severe. Whenever a particular type of fault current If is severe, the circuit breaker 1 should ideally trip to prevent electric power from being supplied to the user""s electric device.
Although the above-described circuit breaker 1 detects fault currents If and blocks the supply of power to the electric device in some instances, it is implemented via analog or digital hardware. Therefore, designing the circuit breaker 1 so that it adequately detects fault currents If, distinguishes the detected fault currents If from among multiple types of fault currents If, and determines the severity of the detected fault currents If, is extremely complex, if not impossible. Furthermore, since the design of the circuit breaker 1 has a complex hardware design, it has to be custom-made for each specific application and standard under which it is to operate. Accordingly, mass-producing the circuit breaker 1 is virtually impossible, and modifying the design of the circuit breaker 1 is very difficult.
In addition, many electrical components that are connected to the conductor network LN typically generate brief, transient leakage currents that are supplied to the electric device for a relatively short period of time. When such transient leakage currents are generated, they will not damage the electric device in the user""s home or cause a fire. However, since the circuit breaker 1 is implemented via hardware, it cannot easily distinguish between harmless transient leakage currents having short durations and damaging fault currents having longer durations. As a result, the leakage currents often cause the circuit breaker 1 to trip and unnecessarily prevent power from being supplied to the electric device. Accordingly, the operational efficiency of the electric device is substantially degraded.
One object of the present invention is to provide a fault current detection system that can detect fault currents, distinguish the detected fault currents from among multiple types of fault currents, determine the severity of the detected fault currents, and determine the frequencies of the different fault currents. A related object is to provide such a fault detection system that is capable of detecting these parameters more accurately than possible heretofore.
Another object of the present invention is to provide a fault current detection system that can be easily mass-produced and that can be easily modified to work in many different environments.
Still another object of the present invention is to provide a fault current detection system that can easily distinguish between harmless transient leakage currents having short durations and damaging fault currents having longer durations.
In order to achieve the above and other objects, a fault current detection software program is provided. The software program is contained in a computer readable medium and includes instructions to perform a routine comprising: (a) determining predetermined characteristics of said fault current based on a detection signal corresponding to said fault current; (b) identifying said fault current as a first type of fault current when at least one of said predetermined characteristics has a first predetermined value; (c) setting a trigger current to a first trigger current value when said fault current is identified as said first type of fault current; and (d) outputting a control signal when said fault current and said trigger current have a predetermined relationship.
In order to further achieve the above and other objects, another fault current detection software program is provided. This software program is also contained in a computer readable medium and includes instructions to perform a routine comprising: (a) determining at least a frequency of said fault current based on a detection signal corresponding to a fault current; (b) determining if said frequency of said fault current is greater than a first predetermined frequency; (c) identifying said fault current as a high frequency fault current when said frequency is greater than or equal to said first predetermined frequency; and (d) when said frequency is less than said first predetermined frequency, identifying said fault current as a waveform fault current.
In order to still further achieve the above and other objects, a fault current detection system is provided. The detection system detects a fault current generated on a conductive path supplying power to an electric device and prevents the fault current from being supplied to the electric device. The fault current detection system comprises: a detector that detects a fault current generated on said conductive path and outputs a corresponding detection signal; a switch that is provided in said conductive path that selectively isolates said electric device from said conductive path corresponding to a fault current; and a controller that inputs said detection signal and determines predetermined characteristics of said fault current based on said detection signal, wherein said controller identifies said fault current as a first type of fault current when at least one of said predetermined characteristics has a first predetermined value, wherein said controller sets a trigger current to a first trigger current value when said fault current is identified as said first type of fault current, and wherein said controller outputs a control signal to said switch to instruct said switch to isolate said electric device from said conductive network when said fault current and said trigger current have a predetermined relationship.
In order to even further achieve the above and other objects, another fault current detection system is provided. The detection system detects a fault current generated on a conductive path supplying power to an electric device and prevents the fault current from being supplied to the electric device. The fault current detection system comprises: a detector that detects a fault current generated on said conductive path and outputs a corresponding detection signal; a switch that is provided in said conductive path that selectively isolates said electric device from said conductive path corresponding to a fault current; and a controller that inputs said detection signal and determines at least a frequency of said fault current based on said detection signal, and wherein said controller determines if said frequency of said fault current is greater than a first predetermined frequency, wherein said controller identifies said fault current as a high frequency fault current when said frequency is greater than or equal to said first predetermined frequency, wherein, when said frequency is less than said first predetermined frequency, said controller identifies said fault current as a waveform fault current.